Abstract Detail

Ultrastructure of the Glandular Secretory Trichomes in Artemisia annua.

Artemisia annua produces the anti-malarial compound artemisinin, which is sequestered within the glandular secretory trichomes found on leaves, flowers and buds. The ultrastructure of the ten-celled glandular secretory trichomes (GSTs) has previously been studied using chemical fixation; however, chemical fixation is prone to artefacts such as distortion of the plasma membrane, disruption of endomembranes, and coagulation of proteins. More advanced fixation techniques, such as high-pressure freezing/freeze substitution (HPF/FS), have been developed to reduce the occurrence of such artefacts. This method was used to re-examine the ultrastructure of A. annua GSTs and to determine if superior tissue preservation would provide new information on the structure and function of GST cells. The apical tier of cells of GSTs are thought to act as transfer cells, while the second and third tiers of cells may be responsible for the production of isoprenoids, such as artemisinin. HPF/FS coupled with light and electron microscopy confirmed that cell wall invaginations exist in the cell walls of the apical cells of GSTs, but only when the cuticle has separated from the cell wall to create a subcuticular space. Hexadecene was superior to sucrose as a cryoprotectant for HPF preservation. The use of sucrose as a cryoprotectant resulted in plasmolysis and osmiophilic inclusions between the outermost cell wall and the plasma membrane of GST cells. Similar inclusions were observed in previous studies involving chemical fixation. However, these were not observed in material using hexadecene as a cryoprotectant, suggesting that the inclusions may be an artefact of fixation. The apical cells of GSTs lack functional chloroplasts, which suggests that other cells within the GST provide isoprenoids or the precursors for isoprenoid biosynthesis. However, abundant plasmodesmata were not observed between the tiers of cells, suggesting that transport of isoprenoids, or their precursors, occurs via an apoplastic rather than symplastic mechanism.